Tridimensional fiber reinforcement of portland cement concrete matrices

ABSTRACT

Steel fibers or cloth made therefrom having a tridimensional configuration for use as a reinforcement for Portland cement concrete matrices is described. At least three wires, approximately equally laterally spaced from each other, skeletally enclose a volume in the approximate shape of a sphere or ellipsoid; the wires are joined to one another where they meet along the axis of the volume enclosed to form the basic tridimensional element termed an &#39;&#39;&#39;&#39;enclosure&#39;&#39;&#39;&#39;. These enclosures may be connected in series by causing the wires to be sufficiently long to form two or more enclosures. One or more of serially connected enclosures may be used as a reinforcing fiber. A continuous fiber is formed by connecting in series a relatively large number of enclosures. A group of parallel, continuous fibers may be laterally connected to form a tridimensional cloth.

United States Patent [191 Naaman Antoine E. Naaman, Cambridge, Mass.

[73] Assignee: Massachusetts Institute of Technology, Cambridge, Mass.

[22] Filed: Aug. 24, 1972 [21] Appl. No.:283,438

[75] Inventor:

[451 Dec. 10,1974

Primary Examiner--Henry C. Sutherland Assistant Examiner-Carl D.Friedman Attorney, Agent, or FirmArthur A. Smith, Jr.; Martin M. Santa;Robert Shaw [5 7] ABSTRACT Steel fibers or cloth made therefrom having atridimensional configuration for use as a reinforcement for Portlandcement concrete matrices is described. At least three wires,approximately equally laterally spaced from each other, skeletallyenclose a volume in the approximate shape of a sphere or ellipsoid; thewires are joined to one another where they meet along the axis of thevolume enclosed to form the basic tridimensional element termed anenclosure. These enclosures may be connected in series by causing thewires to be sufficiently long'to form two or more enclosures. One ormore of serially connected enclosures may be used as a reinforcingfiber. A continuous fiber is formed by connecting in series a relativelylarge number of enclosures. A group of parallel, continuous fibers maybe laterally connected to form a tridimen- 6 Claims, 19 Drawing Figures[52] US. Cl 52/664, 52/600, 52/659, 52/736 [51] Int. Cl. E04c 5/01 [58]Field of Search 52/600, 664, 630, 719, 52/736, 660, 414, 735, 676, 454,740, 733 734; 245/2, 5

[56] References Cited UNITED STATES PATENTS 906,479 12/1908 Whitacre52/733 1,046,913 12/1912 Weakley 52/736 X Sional cloth 1,124,170 l/1915Rechnitz 52/676 2,754,674 7/1956 Mulsbury et a1 52/734 X PATENTED DEC 10I974 sum r01 or 12 FIG.|

FIG-.2

FIG. 3(A) FIG. 3(8) FIG. 3(C) PAIENIEU HEB I0 I974 sum '02 or i2 ZFAILURE m" (I) i Low YIELD 50 DUCTILE STEEL (STAINLESS) 0 I 1 l l l l lo 0 2 1 0.4 0.6 0.8 L0 L2 I I r-- v ELONGATION PER INCHI FIG. 4(A)FAILURE g HIGH YIELD BRITTLE STEEL w 50 b=o.o2o I =0.oI5

o l l l ELONGATION. PER INCH FIG. 4(8) TRIDIMENSIONAL FIBERREINFORCEMENT OF PORTLAND CEMENT CONCRETE MATRICES This invention hereindescribed was made in the course of a contract, No. DAHC l5-67-C-0222,with the Advanced Research Projects Agency.

This invention relates to a fiber configuration for use as areinforcement for a concrete matrix and in particular to steel fibershaving a tridimensional structure for use as reinforcement in a concreteor cement matrix.

'due to discontinuity. Its use in structural elements,

when design justifies economical considerations, is mainly incombination with conventional reinforcement.

Steel fibers whichare conventionally used have diameters ranging from 4to 30 mils, a length between onequarter and 2 inches and a tensilestrength between 100,000 and 400,000 psi. Obtaining a uniform dispersionof the fibers and preventing their segregation appears to be the mostdifficult problem in preparing a composite of these fibers and concrete.Experience with steel fibers has shown that segregation is related totwo reinforcement parameters: the aspect ratio of the fibers and theirfraction volume in the mix. Increasing the aspect ratio or increasingthe fraction volume of the fibers increases tendency to segregation anddestroys the uniformity of the composite.

The use of steel fibers in concrete is mainly limited by thecost/efficiency ratio. The price per pound of steel fibers will likelyremain at least three or four times the cost of conventional reinforcingsteel. It seems, therefore, that steel fiber reinforcement in itsprevious form is not able to compete in applications where conventionalreinforcement is satisfactory.

These prior-art fibers have as their primary function that of being animpact or shock energy absorber. However, a second but not lessimportant function for fibers would be as a tensile bearing componentidentical to conventional reinforcement in reinforced concrete.Experimental data shows that a common description of the failuremechanism of a composite is that, after cracking of the matrix,specimens failed by pulling out of the fibers bridging the fracturesurfaces. There exists a lack of bonding leading to a low failurestrength. The strength of the fiber is inefficiently used. Attempts toimprove this bonding by increasing the length of the fibers results inincreasing the aspect ratio which leads to segregation and not thedesired composite material. Coating the fibers and improving mechanicalbonding by spiralling the fibers or deforming the fibers assist inimproving the performance of the composite but are unable to solve theproblem completely. Some improvement is expected by using closed formfibers such as rings or rectangles; however, segregation remains aproblem.

The tridimensional fibers of this invention can be used as discontinuousreinforcement, as straight fibers are, where the number of basicelements which are serially connected is small and forms a fiber whichis not long relative to its lateral dimensions. Such fibers can be mixedwith the concrete matrix prior to pouring, incurring less segregationproblems than straight fibers. If mixing the discontinuous fibers in thematrix is not desired, the concrete matrix may be poured over the fiberswhich are previously placed in a form, and pile up due to theirtridimensional shape to occupy part or the entire volume of the form.Discontinuous tridimensional fibers are more efficient than straightwires with respect to improvement in tensile strength, toughness,cracking, bonding and ductility.

The tridimensional fibers may also be constructed to form a continuousfiber by serially connecting a group of basic elements whose totallength extends over a substantial portion of the length of the structureof which it is to form the reinforcement. These continuous fibers may beformed into a cloth by spaced wires laterally joining parallel spacedcontinuous fibers. Continuous or cloth tridimensional fibers allow theuse of high strength steel while still achieving ductility throughcrushing of the concrete matrix in which they are embedded when theapplied load is high enough. There is also an increase in the efficiencyof energy absorption, impact resistance, cracking and ductility. Wherecloth is used, there is no need for spacers to laterally and/orvertically separate the continuous fibers.

Tridimensional fibers of the continuous or discontinuous type offersavings in time and labor over conventional reinforceing rods which mustbe secured in place.

Tridimensional fibers also have the advantage over conventionalreinforcing steel that with a relatively simple, inexpensive machinethey can be fabricated onsite from spools of wire.

It is therefore an object of this invention to produce a fiberconfiguration which incurs less segregation problems than prior artfibers if mixed with a concrete matrix prior to pouring or which may, ifdesired, be

placed in a mold prior to pouring the concrete over the fibers.

It is a further object of the invention to produce a fiber which givesthe matrix in which it is placed greater tensile strength, bonding,toughening and ductility than obtainable with prior art fibers.

It is a still further object of the invention to produce a fiber whichis self-spacing, susceptible to on-site fabrication and capable of beingformed as a cloth.

Other objects and features of the present invention will be apparentfrom the following description of tridimensional fiber elements read inconjunction with figures in which FIG. 1 shows one embodiment of thetridimensional fiber element, identified as the type 1 element;

FIG. 2 shows an alternate embodiment of the tridimensional element,identified as the type 2 element;

FIG. 3(a) shows a first mode of reinforcing concrete where thetridimensional elements are uniformly disposed;

FIG. 3(b) shows a second mode of reinforcing concrete wheretridimensional elements are concentrated in the tensile stress area ofthe concrete;

FIG. 3(0) shows a third mode or reinforcing concrete where thetridimensional elements are in a single layer just covered by theconcrete;

FIG. 4(a) is a graph of stress versus strain for a low yield strengthductile steel fiber;

FIG. 4(b) is a graph of stress versus strain for a high yield strengthbrittle steel fiber;

FIG. 5 is a graph of load versus elongation for conventionalfiber-reinforced concrete under tensile load- FIG. 6 shows graphs ofload versus deflection for conventional fiber-reinforced concrete underflexural loading;

FIG. 7 shows graphs of load versus deflection for a type 1tridimensional fiber element reinforced concrete using the first modeofreinforcement with ductile steel;

gation for a type 1 tridimensional fiber element-- reinforced concreteusing the third mode of reinforcement with different wire diameters andvolume fraction of ductile steel;

FIG. 10 are graphs of load versus deflection of a type 2 tridimensionalfiber element-reinforced concrete I using the first mode ofreinforcement with ductile steel;

FIG. 11 shows graphs of load versus deflection for a type 2 continuoustridimensional fiber elementreinforced concrete using the second mode ofreinforcement with ductile steel;

FIG. 12 shows graphs of load versus deflection for a type 2 continuoustridimensional fiber elementreinforced concrete using the second mode ofreinforcement with brittle steel;

FIG. 13 shows a graph of load versus deflection for a type 2 continuoustridimensional fiber elementreinforced concrete using the second mode ofreinforcement with brittle steel and incompletely cured concrete; and

FIG. 14 shows continuous fibers attached to each other to form a cloth.

The tridimensional fiber of the skeletal type has the property that thebond strength of the steel in concrete is increased by a fiber of thisform by adding a mechanical component to bond. Tridimensional fibers mayhave a great variety of shapes, for instance, that of a tetrapode orspherical skeleton. Two preferred embodiments are shown in FIGS. 1 and2, where the skeletal arrangement of the wires approximate an ellipsoid.FIG. 1 shows four wires 10 equally spaced from each other to form theskeletal outline ll of an ellipsoid. The wires are twisted together atthe ends l2, 13 of the ellipsoid; and, in the embodiments shown in FIGS.1 and 2, the wires are continued to form another element 14 identical tothat of element 11. Together, elements 11 and 14 form a fiber designatedas type 1 fiber in the test results to be subsequently presented.

An alternate embodiment of the tridimensional element is shown in FIG.2, where only three equally spaced wires 20 form the ellipsoidalskeleton, and the fourth wire 21 joins the others where they are twistedtogether at the ends 12, 13 of the ellipsoid to form an element 22.Elements 22 and 23 are joined to each other to form a type 2 fiber asdesignated in the test results to be subsequently presented.

It is apparent that tridimensional fibers may be fabricated of as littleas one of the elements 11 or 22 or may be formed of a large number ofserially connected elements. In this application, test results aredisclosed for fibers having two elements which have been designated asdiscontinuous fibers and for fibers having eight elements, which havebeen designated as continuous fibers.

The type 1 and 2 elements used to perform the tests disclosed in thisapplication had typical respective dimensions a z 3.2 to 3.5 inches and3 inches, b l inch and 0.75 inches and c z 0.5 inches, where a, b and care the dimensions illustrated on FIGS. 1 and 2. To ascertain the effectof wire diameter on the strength of the concrete matrix in which theelements were embedded, wire diameters ranging from 0.012 to 0.032inches were used. The wire size for each element tested is given on thefigures. Larger wire sizes than those used appear desirable but were nottested because the machine used in fabricating the elements wasincapable of handling larger wire sizes.

Besides the higher bonding properties of three dimensional fiberelements in comparison to the ordinary steel fiber, a second advantageis that the fibers may be thrown into a mold before pouring in theconcrete matrix because the elements pile up to occupy a volume intowhich the concrete matrix may penetrate. The concrete matrix may haveaggregate limited in size to the openings of the element, andpenetration into the element may be assisted by conventional vibrationtechniques. Therefore, there is considerable freedom in designing themix, and the risk of segregation is eliminated.

A third advantage of the three dimensional fiber elements relates totheir volume-enclosing shape which, upon high enough elongationstresses, tends to cause crushing of the enclosed matrix, which producesa ductile type of failure even though the fibers themselves may bebrittle steel of high yield strength. Also, three dimensional fibers actas crack arrestors and toughening agents because of the enormous lateralbonded area they offer the surrounding matrix.

Variations in the shape of the links and the methods of joining thewires along the fiber axis between links are left to the choice of thedesigner. For example, joining may be obtained by twisting as inpreferred embodiments, or by clamping, welding, gluing, etc. Twistingthe wires is expected to provide a lower apparent modulus and higherelongations (by untwisting) but should be done uniformly as otherwisethe strength of the twisted strand formed would be drastically reduced.

Three methods of using the fiber elements of FIGS. 1 and 2 have beeninvestigated and are presented shcematically in FIG. 3. Mode 2 shown inFIG. 3(b) is in some way similar to conventional reinforcement: throwingin a certain amount of fibers which, when ultimately embedded in thematrix. will form the tensile load carrying element. This procedure isvery easy, for example, in simply supported beams where reinforcement isneeded in the bottom region.

Mode 1 shown in FIG. 3(a) provides the isotropic crack arrestingmechanism and toughening effect of conventional fiber reinforcedconcrete throughout the member being reinforced. Mode 3 of FIG. 3(0) isuseful for relatively thin elements where reinforcing action is needednear both faces and for which fibers may be especially shaped. Slabs,wall panels, shells and pavements are typical applications. This modemay also be attractive for prestressed pavement by means of expansivecements because tridimensional steel fiber may create a very effectivetriaxial restraint necessary to the prestressing action. In general,what is true for tridimensional fibers holds similarly for strand orcloth made of three dimensional fibers. I

The tridimensional fiber has the advantage that the element is wellanchored to the concrete matrix by the skeletal shape of the fibers.Transfer of load from fiber to fiber is obtained by shearing of thematrix in between overlapping links. On the other hand, if used as acontinuous reinforcement, it has the advantage of increasing the bondstrength and efficiency of reinforcement against cracking. I

If the appliedload is high enough, failure can occur through one or acombination of the following situations: (a) the fibers, ifdiscontinuous, pull out; but, due to their bonding properties, thisoccurs at a much higher load than with straight fibers, everything elsebeing equal; (b) the wires forming the fiber reach their yielding pointand lead to the failure of the member; (c) the matrix volume, sustainingthe load transfer between overlapping discontinuous fibers, reaches itsfailure point; (d) the matrix inside the volume enclosed by fiberreaches its failure point under complex multiaxial stresses.

It is a priori easy to imagine that, for a combination of specificvariables, ductile failure of a three dimensional fiber-reinforcedmember may be expected at a relatively high enough stress. For example,for failure as in (b), it is sufficient that the wire be ductile. In thelast two cases, (c) and (d), it seems necessary that the load carried bythe fiber is higher than some particular value. beyond which the matrixwill be crushed, allowing further elongation of the fiber and thereforeductility for the member. In other words, a combined action is expectedfor ductile behavior.

The testing program consists of two major parts: the first one dealswith specimens reinforced with straight steel fibers; and, forcomparison, the second one uses the two types of tridimensional fibersof FIGS. 1 and 2.

Matrix Composition Essentially the same mortar matrix was usedthroughout the test program. High early Portland cement type III andfine graded Ottawa silica sand(ASTM C-l09) were mixed with water in thefollowing proportions: water to cement ratio =O.6; sand to cement ratio2.5.

For the tests involving tridimensional fibers, the fibers were throwninto the empty mold and then the mortar matrix was poured. A slightvibration was applied to allow good penetration. However, using theseventh day, except for series G which was tested before curing, i.e.,tested 24 hours after pouring. Prior Art Straight Steel Fibers Forpurposes of comparison with the tridimensional fiber elements of theinvention, tests were made of straight steel fiber reinforced concretewith parameters within average values recommended by most previousinvestigators in order to avoid experimental obstacles, mainlysegregation of the fibers, etc. Typical load elongation curves fortensile and flexural specimens reinforced with straight steel fibers areshownin FIGS. 5 and 6, respectively. The fibers used were high yield,brittle steel, as in FIG. 4(b), 0.75 inches in length and 0.016 inchesin diameter, uniformly mixed in the concrete matrix. The fractionalvolume of fibers is shown on the curves of the figures. It is seen thathigher fractional volumes of fiber produces higher peak load capacityand greater energy absorption which is measured by the area under theload-elongation or loaddefiection curves.

Tests of Tridimensional Steel Fibers The second part of the test programconsists of several series of single tests under flexural loading,except for series C which was tested under tension. Both types oftridimensional fibers described in FIGS. 1 and, 2 were used under thethree reinforcing modes indicated in FIG. 3: namely, mode 1 and 2 forflexure, mode 3 for tension.

Two types of steel wires were used for the tridimensional fibers testedin this study: stainless steel with low yield stress and very highductility, carbon steel with high yield and low ductility. Typicalstress elongation curves are illustrated in FIG. 4.

For series A, B and C, tridimensional discontinuous fibers, the type 1element of FIG. 1 and stainless steel low yield ductile wire were usedunder the three reinforcing modes already mentioned. Series A, FIG. 7,corresponding to the first reinforcing mode of FIG. 3(a) was made up ofthree specimens having different volume fractions of fibers (obtained byusing different wire diameters). Fibers were thrown into the mold wherethey piled up filling it completely. Then, the mortar matrix was poured,and a slight vibration applied to insure good penetration.

Series B, FIG. 8, corresponding to the second reinforcing mode of FIG.3(b), consisted of only one specimen in which the fibers were throwninto the lower half part of the beam in order to form the tensile loadcarrying element (similarly to conventionally reinforced structures).

Series C, FIG. 9, investigated the third reinforcing mode of FIG. 3(0)by working with tensile plates. One layer of fibers was thrown into themold, and then just enough mortar was poured to cover it completely.Thickness of the specimen was recorded before testing. The threesepcimens C1, C2 and C3 of FIGS. 9(a), (b) and (c), respectively, hadthe following dimensions: 2 X 0.625 X 12, 2 X 0.825 X I2 and 2 X 0.75 X12 inches.

The series D, FIG. 10, experiments were mainly intended to repeat seriesA using the FIG. 2 type of tridimensional discontinuous fibers. However,due to difficulties in producing the fibers, smaller wire diameters werechosen.

Series E, FIGS. ll, 12 and G, FIG. 13, were performed in order toinvestigate the behavior of specimens reinforced with tridimensionalcontinuous fibers of type 2 elements. Each specimen was reinforced with20 strands thrown in the lower half of the beam. Series E was dividedinto two parts using the two types of steel mentioned: low yield, highductility, FIG. 11, and high yield, low ductility, FIG. 12. Strength andductility of the reinforced members had to be compared. As the ductilityof specimens E3 and E4 (reinforced with high strength strands) was quitelow, it was necessary to investigate the mutual interaction between theload carried by the fiber and the matrix. It would have been necessaryto use higher diameter wires to investigate the possibility of a ductiletype of failure due to a crushing of the matrix within the volumeenclosed by the element. As it was difficult, with the availableequipment, to produce tridimensional strands of higher diameters, thestrength of the matrix was reduced instead. Therefore, specimen G wasprepared with the tensile reinforcement as specimen E4 and with the samematrix composition, but was tested 24 hours after pouring, when thestrength of the matrix would have been 20 to 30 percent of the expectedstrength at 7 days. The failure of specimen G was much more ductile thanE4 proving that it is possible to obtain ductile failure by a combinedinteraction between fibers and matrix. Testing Procedure All testing wasperformed using an INSTRON Universal testing machine which applied loadat a rate of 0.05 inch/minute for all the flexural and tensile tests.Load-deformation curves were automatically plotted on a chart recorder.These deformations, representing deflections or elongations, are the sumof the specimen deformation and that of the testing system. Their valueis, therefore, comparative but not absolute. The load deformation curvesare shown in FIGS. 543. Discontinuous Reinforcement Considering firtseries A, B, C and D which involved the use of tridimensionaldiscontinuous fibers of either type I or 2 made with ductile steel,FIGS. 7l0, it is observed that higher ultimate stresses, more drasticcrack propagation and more significant ductile failure and so energyabsorption was obtained than with straight steel fibers as shown inFIGS. and 6. Even at relatively low percentages of reinforcement, thepost-cracking load was higher than the first crack load and yield typebehavior occurred before failure. The drop in the loaddeformation curveafter maximum load was in general very smooth, creating a kind ofplateau, typical of good ductile behavior and energy absorption. Thisoccurred even for specimen B, which was only partially reinforced as inconventional beams. The number of cracks in the middle of the flexuralbeams was significantly higher, at least 5, compared to one crack thatdeveloped in a similar specimen reinforced with straight fibers. Thisresults from the higher efficiency of tridimensional fibers due to theirlength, bonding properties, higher directionality and geometrical shape.

Comparing the load curves of FIGS. 7-10 with respect to load andductility, we can observe betterresults in series A, reinforced withtype 1 fibers, than in series D, reinforced with type 2 fibers. A prioriit had been expected, at least with respect to strength, that the type 2fibers would be better. However, it was observed that, in series D, themain crack occurred and opened along a section where a big number offiber ends were visible. The presence ofa higher concentration of endsin a cross section is due to the end plate effect of the molds. In orderto reduce this effect, a V- shaped piece of wire mesh was placed at eachend of the mold in series A, B and C. This procedure was not followedfor series D. However, this sort of effect would be automaticallyeliminated in a member of larger size, or greatly reduced by usingdiscontinuous fibers of different sizes. Results of tensile tests inseries C are somehow difficult to compare with those obtained withstraight fibers, due mainly to differences in the geometry of thespecimens and the testing apparatus used. Qualitatively, however, FIG. 9shows an improved ductile type of failure for tridimensional fibers aswell as high cracking and disruption of the matrix close to the majoropening crack.

Comparing FIGS. 6 and 7, we can easily see that the energy absorbed atfailure, taken as area under the load deflection curve, in series A(tridimensional fibers) FIG. 7, as an example, is one order of magnitudehigher than a similarly reinforced specimen of straight fibers, FIG. 6.This trend is general, as shown in the other series of experiments,FIGS. 8-10. It should be noted, however, that the ductile property ofthe steel has certainly also considerable influence on the ductilefailure observed in these tests.

No shear failure was observed in specimens uniformly reinforced (Mode 1,series A-D) indicating that tridimensional fibers, similar to straightfibers, do enhance the shear resistance of the concrete matrix.Continuous Reinforcement At failure the tridimensional strand reinforcedbeams of series E of FIG. ll, 12 and G of FIG. 13 showed extensive crackpropagation in the region of the constant moment. In series E, specimensE3 and E4, FIG. 12, reinforced with high yield, brittle steel, showedmuch lower ductility at failure than specimens El and E2, FIG. 11,reinforced with low yield, ductile steel, while a similar order ofstrength was observed for the same fraction volume of reinforcement, Elversus E4. These results indicate that the steel properties are of majorimportance; and that, in this particular series of tests, ductility ofthe steel leads to a ductile type of failure. In general, failureoccurred progressively with the fracture of the reinforcing fibers byone or more of their wire components.

From the results in series E, it is deduced that, for the particularcharacteristics of the components used, the ductility of the steeldetermined whether there was ductile behavior of the member beforefailure. As it was mentioned earlier, specimen G of FIG. 13, having thesame tensile reinforcement as E4, was meant to investigate thepossibility of a combined action between tridimensional fiber and matrixwhich will result in a ductile type of failure. This action was obtainedby testing specimen G just 24 hours after pouring, at which time thecompressive strength of the matrix was relatively low. Under appliedtensile stresses the skeletal part of the tridimensional fiber tends tostretch, crushing the inner, weak concrete matrix and so allowing formore extension and ductility. The improvement in ductility can beobserved by comparing the load deflection curves of specimens E4 of FIG.12 and that of FIG. 13.

In summary of the test results, it can be said that, on the basis ofstrength, tridimensional continuous reinforcement gave, in general,lower values than theoretically predicted using A.C.I. ultimate momentformula for reinforced concrete; while, on the basis of crackingpropagation, ductility and energy absorption, drastic improvements overconventional reinforcement have been achieved.

What is claimed is: l. A reinforcing fiber for a concrete matrixcomprising a plurality of three or more steel wires, said wires forminga skeletal outline of a volumetric enclosure, said wires beingsubstantially equally spaced from each other on said enclosure, saidwires being joined together at opposite ends of the enclosure, anadditional steel wire through the center of said volumetric enclosure,joining the other skeletal wires at the ends of the enclosures. 2. Thereinforcing fiber of claim 1 comprising, in addition,

a plurality of said fibers, said fibers being serially connected at theends where said wires are joined together to form a continuous fiberhaving a plurality of enclosures.

a plurality of wires substantially parallel and transverse to thedirection of said continuous fibers, each wire of said plurality beingconnected to a continuous fiber to form a grid-like fiber cloth. 6. Thefiber of claim 2 wherein said joined wires are wires that are twistedabout each other.

l l l =l

1. A reinforcing fiber for a concrete matrix comprising a plurality ofthree or more steel wires, said wires forming a skeletal outline of avolumetric enclosure, said wires being substantially equally spaced fromeach other on said enclosure, said wires being joined together atopposite ends of the enclosure, an additional steel wire through thecenter of said volumetric enclosure, joining the other skeletal wires atthe ends of the enclosures.
 2. The reinforcing fiber of claim 1comprising, in addition, a plurality of said fibers, said fibers beingserially connected at the ends where said wires are joined together toform a continuous fiber having a plurality of enclosures.
 3. Thereinforcing fiber of claim 2 wherein said wires forming a fiberenclosure are continued without being broken and formed into seriallyconnected fiber enclosures called a continuous fiber.
 4. The reinforcingfiber of claim 3 wherein the wires forming the fiber enclosures aretwisted about one another in the region between the enclosures and atthe ends of the continuous fiber to achieve the enclosures''connections.
 5. A reinforcing cloth comprising a plurality of continuousfibers as in claim 2 arranged in a parallel relation and spaced fromeach other, a plurality of wires substantially parallel and transverseto the direction of said continuous fibers, each wire of said pluralitybeing connected to a continuous fiber to form a grid-like fiber cloth.6. The fiber of claim 2 wherein said joined wires are wires that aretwisted about each other.